This invention relates to a method and system for on-line sensing of cutting tool condition, such as wear and breakage, by monitoring heat generated by the cutting process.
The quality and accuracy of a machined part is directly related to the condition of the cutting tool and its cutting edge. A cutting tool wears during normal machining operations, and after a prescribed amount of wear it is rendered unsuitable for use. Its continued use may lead to an unacceptable part size, surface finish, and chatter and could lead to tool fracture which jeopardizes the integrity of the workpiece and machine tool. Thus, most numerically-controlled operations require that a prior determination be made of the expected tool life. Tool usage is then programmed not to exceed the allotted time.
Due to inherent tool material, workpiece, and process variations, any prescribed tool life has to be a conservative estimate to minimize the probability of excessive wear on tools. With this operational approach, especially in situations where tool life is limited such as the machining of titanium, the tooling costs will be unnecessarily high. The problem to be solved is the measurement of tool condition during the machining process, and the use of this measurement in the utilization of individual tools to their maximum capability without direct human intervention.
There are a number of systems, based on utilizing different sensors, such as force, acoustic emission, vibration, machine tool feed and spindle drive currents, and tool temperature, that have been advocated as tool condition sensors. Although claims abound that they are successful, there remains a dearth of sensors applicable for the on-line sensing of tool condition. Acoustic tool break detectors, such as the machine tool monitor described in copending application Ser. Nos. 664,188 and 664,189 filed Oct. 24, 1984, now U.S. Pat. Nos. 4,636,780 and 4,636,779, C. E. Thomas et al, address the problem of tool breakage under limited conditions. Systems based on the measurement of machining force have been partially successful for tool wear and tool break sensing but are too costly for routine application and generally require large modifications to the machine tool. One such system has been shown to suffer from long-term reliability due to the wear of bearings through which machining force is measured. Systems based on the measurement of machine tool drive currents, although inexpensive, are difficult to use and are more appropriate for use in transfer-line situations than in flexible manufacturing systems.